Electrically driven ceramic oxygen generators and oxygen separators are used to separate oxygen from an oxygen containing feed stream, for example, air. Electrically driven ceramic oxygen generators or oxygen separators can utilize planar or tubular ceramic membrane elements having a layered structure containing an electrolyte layer capable of transporting oxygen ions when subjected to an elevated temperature, cathode and anode electrode layers located at opposite surfaces of the electrolyte layer and current collector layers to supply an electrical current to the cathode and anode electrode layers.
When the tubular membrane elements are subjected to the elevated temperature, the oxygen contained in a feed will ionize on one surface of the electrolyte layer, adjacent the cathode electrode layer by gaining electrons from an applied electrical potential. Under the impetus of the applied electrical potential, the resulting oxygen ions will be transported through the electrolyte layer to the opposite side, adjacent the anode layer and recombine into elemental oxygen.
The tubular membrane elements are housed in an electrically heated containment or enclosure to heat the tubular ceramic membrane elements to an operational temperature at which oxygen ions will be transported. Additionally, such tubular membrane elements can be manifolded together such that the oxygen containing feed is passed into the heated containment and the separated oxygen is withdrawn from the tubular membrane elements through a manifold. In certain purification applications, the oxygen containing feed can be passed through the interior of the tubular membrane elements and the separated oxygen can be withdrawn from the containment.
Typical materials that are used to form the electrolyte layer are yttrium or scandium stabilized zirconia and gadolinium doped ceria. The electrode layers can be made of mixtures of the electrolyte material and a conductive metal, a metal alloy or an electrically conductive perovskite. Current collectors in the art have been formed of conductive metals and metal alloys, such as silver as well as mixtures of such metals and metallic oxides.
The tubular membrane elements can be contained in one or more modules in which in each module, the tubular membrane elements are arranged in bundles and are held in place by end insulation members adjacent to the opposite ends of the tubular membrane elements. These modules can be positioned within insulated, heated enclosures to heat the tubular membrane elements to an operational temperature at which oxygen ion transport can occur. The insulated enclosure also has inlets and outlets within end walls of the enclosure to allow an oxygen containing feed stream to be passed into the enclosure and thereby to contact the tubular membrane elements. As a result of the oxygen separation, a retentate stream is formed that is discharged from the enclosure through the outlet. This type of electrically driven oxygen separation device is shown in U.S. Patent Application Publication No. 2010/0116680 A1.
As can be appreciated, it is important that electrically driven oxygen separation devices reliably deliver high purity oxygen and at the lowest cost possible. With respect to reliability, a major problem with electrically driven oxygen separation devices is that failure of the tubular membrane elements can occur. As a result, the oxygen containing feed stream will pass through the point of failure in a particular tubular membrane and little if any oxygen will be separated by the membrane that has the defect. Since a major advantage of supplying oxygen from an electrically driven oxygen separation device is that the oxygen can be produced at ultra-high purity, the defective tubular membrane element will result in an unacceptable decrease in purity of the oxygen product. Therefore, as a result of such failure, the electrically driven oxygen separation device will have to be removed from service. Furthermore, such a device is most useful if the outlet of oxygen separation modules are connected to a storage tank and the oxygen is stored at pressure. In the case of a tube failure, the stored oxygen in the tank will discharge through the fractured ceramic tube. In order to reduce costs, the electrically driven ceramic oxygen generator has to be assembled in a cost efficient manner. In the patent application discussed above, the use of a plurality of modules of such ceramic membrane elements coupled with polymeric end seals go a long way toward reducing assembly costs. However, such ends seals represent another possible point of failure because they have only a limited ability to withstand the high temperatures that are necessary to induce the oxygen ion transport in the tubular ceramic membrane elements.
When operated in a continuous mode of oxygen generation, a properly designed ceramic oxygen generator is capable of producing very high purity oxygen. However, during normal operation, the ceramic oxygen generator will typically experience idle periods often dictated by the end-user oxygen supply requirements. It has been observed that during such idle periods, the seal materials used within the typical ceramic oxygen generator become compromised, thereby allowing the high purity oxygen to leak from the ceramic modules and/or allow nitrogen or other contaminants to diffuse into the modules and ceramic membrane elements. In addition, the ceramic modules and associated manifolds used in typical ceramic oxygen generators contain a significant number of fittings that are also potential sources of leaks.
As will be discussed, the present invention provides an electrically driven ceramic oxygen generator device that, among other advantages mitigates the oxygen purity concerns arising during idle mode and is capable of ensuring the prescribed purity level of high purity oxygen is always delivered to the receiving tank.